Retinal photoreceptors sit at the interface between the brain and the visual world. Their task is central to vision: the translation of light energy into an electrical signal that can be passed along and analyzed by other cells in the nervous system. An understanding of how photoreceptors accomplish this vital task is requisite for understanding of human vision, and a number of experiments in the proposed research plan will focus on the mechanism of generation of the electrical signal within the photoreceptors. Fundamental properties of individual ion channels that underlie the electrical signal will be studied, as well as the gating of the channels by the intracellular chemical messenger, cyclic guanosine monophosphate, that is modulated by light. In these experiments, a sensitive electrical recording technique will be used to measure the ionic current flowing through a single open channel, either in an intact cell or in an excised patch of plasma membrane. Another important step in vision is the transmission of the electrical signal in the photoreceptors to other neurons in the retina; this occurs via chemical synaptic transmission, but the identity of the neurotransmitter released by the photoreceptors is unknown. Other experiments will be aimed at the identification of this neurotransmitter released from the synaptic terminals of both rod and cone photoreceptors.